1. Field of the Invention
The present invention relates to a zoom lens system more particularly, but not exclusively, a zoom lens system that can be operatively connected to an image device.
2. Description of the Related Art
Hitherto, in the field of zoom lenses used for photographic cameras and camcorders, various zoom lenses using a rear focus system have been proposed. In the rear focus system, focusing is performed by moving a lens unit on the image side (opposite to the object side) of a first lens unit.
A conventional rear-focus zoom lens includes five lens units whose refractive powers are positive, negative, positive, negative, and positive, from the object side to the image side.
Japanese Patent Laid-Open No. 05-119260 (corresponding to U.S. Pat. No. 5,528,427) discusses a zoom lens including, from the object side to the image side, a first lens unit L1 having positive refractive power, a second lens unit L2 having negative refractive power, a third lens unit L3 having positive refractive power, a fourth lens unit L4 having negative refractive power, and a fifth lens unit L5 having positive refractive power, as shown in FIG. 11. The fifth lens unit L5 includes, from the object side to the image side, a lens G51 having positive refractive power, another lens G52 having positive refractive power, and another lens G53 having negative refractive power. Aberrations such as spherical aberration, coma aberration, and distortion generated in the fourth lens unit L4 are reduced by generating aberrations such as spherical aberration, coma aberration, and distortion in an air lens between the positive lens G52 and the negative lens G53. Thus, improved optical performance can be achieved throughout the entire zoom range.
Although this zoom lens has a simple lens configuration and improved optical performance, it has no image-stabilizer lens unit.
An image-stabilizer lens unit can have negative refractive power, which can reduce the diameter of the product including it.
Therefore, in the case of this zoom lens, the whole or part of the second lens unit L2 or the fourth lens unit L4 having negative refractive power can be an image-stabilizer lens unit.
However, if the second lens unit L2 is an image-stabilizer lens unit, the outside diameter of the image-stabilizer lens unit is large. This does not aid compactness. In addition, if the second lens unit L2 is an image-stabilizer lens unit, it is difficult to use the second lens unit L2 as a focusing unit.
On the other hand, if the fourth lens unit L4 is an image-stabilizer lens unit, the second lens unit L2 can be used as a focusing unit. In addition, the fourth lens unit L4 has the smallest lens diameter of all the lens units. There is no problem in terms of compactness.
However, in the image-stabilizer lens unit, the stabilization can be facilitated by reducing the coma aberration as well as the decentration aberration.
The coma aberration generated in the fourth lens unit L4 can, at least partially, compensate for the coma aberration generated in the fifth lens unit L5. Therefore, if the coma aberration generated in the fourth lens unit L4 is reduced, one can reduce the coma aberration generated in the fifth lens unit L5 accordingly.
That is to say, one can reduce the refractive power of the air lens between the lenses G52 and G53. Consequently, the difference between the radius of curvature of the image-side surface Ra of the lens G52 and the radius of curvature of the object-side surface Rb of the lens G53 can be small.
When the difference between the radiuses of curvature of the surfaces Ra and Rb, which form the air lens in the fifth lens unit L5, is small, the problem of a ghost image can occur.
That is to say, light travels in the optical path shown in FIGS. 12 and 13 (undergoes surface reflections), which generates a ghost image. FIG. 13 is an enlarged view of part A of FIG. 12.
As shown in FIG. 13, light LG undergoes a multiple reflection between the surfaces Ra and Rb, is incident on the image plane, and becomes ghost light.
Depending on the position of a light source, the angle of incidence of the ghost light on each surface of the air lens can be more than 50 degrees. Therefore, reflectance is increased. An intensity of ghost light still exists at appreciable levels even if the surfaces Ra and Rb have an antireflection coating.
The air lens formed by the surfaces Ra and Rb can reduce the distortion as well as the coma aberration. If the refractive power is small, it is difficult to reduce the aberrations.